Conventionally, semiconductor light emitting devices including semiconductor light emitting elements as light sources have been known. Generally, for such semiconductor light emitting devices, a light emitting element (light emitting diode element) using a nitride-based semiconductor is mounted. A semiconductor light emitting element using a transparent substrate of, for example, sapphire substrate, has been known as an example of the light emitting element using nitride-based semiconductor. In such a semiconductor light emitting element, a multi-layered film of nitride-based semiconductor including a light emitting layer is formed on the transparent substrate. On the multi-layered film, typically, an electrode layer including a light transmitting electrode and a pad electrode is formed.
In a semiconductor light emitting element, light emitted downward from the light emitting layer enters the transparent substrate and is reflected from the back surface side of the substrate. The light reflected from the back surface side of the substrate returns to the upper part of the semiconductor light emitting element, and part of the light enters the semiconductor multi-layered film. The light that has entered the multi-layered film passes through the multi-layered film and the like and is taken out to the outside of the light emitting element. Part of the light, however, is absorbed, for example, by the light transmitting electrode, the pad electrode and the light emitting layer. Therefore, light extracting efficiency is higher when the light reflected from the back surface side of the substrate is taken out from a side surface of the transparent substrate than when the light is taken out from the upper surface side (on which the multi-layered film is formed) of the light emitting element.
By way of example, assume that a sapphire substrate is used as the transparent substrate and the light is directly extracted to the air from a side surface of the sapphire substrate. Here, the angle of total reflection (θside: if the light is incident on the side surface of substrate with this or larger angle with respect to the vertical direction, the light is totally reflected) at the interface between the sapphire substrate (refractive index=1.78) and the air (refractive index=1) is θside≧34.18°. Specifically, consider the light emitted downward from the light emitting layer and entered the sapphire substrate. Of the light beams directly proceeded to or reflected by the back surface of sapphire substrate and directed to the side surface of sapphire substrate, those incident on the side surface of sapphire substrate with the angle of 34.18°≦θside≦90° with respect to the vertical direction of the substrate side surface are not taken out from the side surface of sapphire substrate but returned toward the side of multi-layered film of nitride-based semiconductor including the light emitting layer formed on the sapphire substrate. On the other hand, light beams with the incident angle of θside<34.18° are emitted to the air from the side surface of sapphire substrate.
A semiconductor light emitting element is generally mounted on a stem or the like and sealed with transparent resin having the refractive index of about 1.4 to about 1.5. Here, the difference in refractive index between the transparent substrate and the transparent resin is smaller than the difference in refractive index between the transparent substrate and the air. Therefore, total reflection of light at the side surface of transparent substrate is less likely than when the side surface of transparent substrate is in contact with the air. As a result, it becomes easier to extract light with higher efficiency from the side surface of transparent substrate.
By way of example, assuming that the sealing resin has the refractive index of 1.5, the angle of total reflection at the interface with the side surface of sapphire substrate is θside≧57.43°. Specifically, consider the light emitted downward from the light emitting layer and entered the sapphire substrate. Of the light beams directly proceeded to or reflected by the back surface of sapphire substrate and directed to the side surface of sapphire substrate, those incident on the side surface of sapphire substrate with the angle of 57.43°≦θside≦90° with respect to the vertical direction of the substrate side surface are not taken out from the side surface of sapphire substrate but returned toward the side of multi-layered film of nitride-based semiconductor including the light emitting layer formed on the sapphire substrate. On the other hand, light beams with the incident angle of θside<57.43° are emitted to the transparent resin from the side surface of sapphire substrate. In this manner, by sealing the semiconductor light emitting element with the transparent resin, it becomes possible to extract larger amount of light from the side surface of sapphire substrate. It is noted, however, that still some amount of light is totally reflected at the side surface of sapphire substrate. Therefore, it is necessary to further improve the light extraction efficiency to minimize the totally reflected light.
Patent Literature 1 specified below proposes, as a solution to such a problem, to form irregularities on the back surface of transparent substrate. According to Patent Literature 1, light beams emitted downward from the light emitting layer, incident on the sapphire substrate, mirror-reflected by the back surface of sapphire substrate and again returned to the side of light emitting layer come to be reflected at angles different from those of the conventional examples because of the irregularities and, hence, it becomes easier to extract light from the side surface of substrate. According to Patent Literature 1, if the back surface of transparent substrate is in contact with air, there is a large difference in refractive index and, therefore, remarkable light scattering effect can be attained by the structure with irregularities. This enables improved efficiency of light extraction to the outside.